The radiation response of the fine vasculature may be important to the functional status of dependent tissues after a course of radiation therapy. The stress of additional functional or traumatic demands on radiation damaged vasculature may have serious life-threatening consequences. Therefore it is imperative to study the inherent radioresponsiveness of microvasculature. This is especially important with the use of new modalities such as hyperthermia or chemotherapy in conjunction with radiation. The primary objective of this research is to determine the radioresponse of slowly proliferating capillary endothelial cells. A model system involving the controlled stimulation of capillary growth into the avascular cornea has been developed for the dog. Morphometric methods have been devised to quantify this response. Studies concerned with long term (months) radiation effects on microvasculature after single and fractionated treatments with or without certain chemical adjuvants or hyperthermia can be performed. The results can be correlated with studies of the vascular response of the heart, as monitored by electrocardiography, echocardiography, radiography and morphometric analysis.